Foam insulation on sloped roof?

Every energy audit site now says when you reroof you should add foam insulation to the roof to help keep your attic cooler. I even had a home auditor come out and tell me the same thing, but no one will tell me what this means exactly.

I even tried to look it up, and found no information, just people doing sprayfoam on the inside of the roof.

Does this mean ripping up the shingles, laying 2" of XPS foam then reshingleing? that would be too soft wouldn't it? or does it mean put foam on the roof deck, then deck over the foam, then reshingle? or is it talking about on the inside of the attic space?

if its on the inside, how would you do that with a finished cathedral ceiling?

our "vented" attic is 150 degrees.... it's your standard soffit vent with vent baffels to keep the insulation from plugging it up type the soffit vents

problem is that 150 degree attic is making the second floor very hot, even with adequate air flow to the 2nd floor from the Central AC (CFM was verified as what the room should need at the registers)

that's where the auditor that came out said to insulate the roof to "fix" the issue....

the attic is already at R-40 with blown fiberglass...

one energy auditor wanted to remove the fiberglass and replace it with blown celulose to reduce air flow and heat transfer since it has a higher R value...

the part of the attic over the 2nd story part is only 4 ft high at the highest point I think it was, so the roof is pretty close to the room's ceiling level, even fully vented, one vent per joist cavity on the roof with the baffels and such, doesn't seme to help much.

I am putting in solar fan's to see if that helps, haven't goten to it yet though...

the odd thing is we have a 1 story master suite that doesn't get as hot as the 2nd floor... but it has a full attic witch is almost 13ft high at the peak but it doesn't get anywhere near as hot as the 2nd story attic space.. the hottest it was, was 110 degrees at the same time the 2nd story space was 150 degrees... and heat doesn't transfer in that space to the master suite like the 2nd floor one does... same type of blown in insulation also

our "vented" attic is 150 degrees.... it's your standard soffit vent with vent baffels to keep the insulation from plugging it up type the soffit vents

problem is that 150 degree attic is making the second floor very hot, even with adequate air flow to the 2nd floor from the Central AC (CFM was verified as what the room should need at the registers)

If the temperature is that far off from ambient, it is not vented proper or something else is exacerbating the heat issue. A properly vented attic should only be about 20-25 degrees off of ambient in a sloped roof (4:12 or better)

Quote:

Originally Posted by BlueBSH

that's where the auditor that came out said to insulate the roof to "fix" the issue....

Your auditor might be a idiot in this case because that is about the most round about way to reduce heat in the attic.

Quote:

Originally Posted by BlueBSH

the attic is already at R-40 with blown fiberglass...

Worst stuff on the market. Usually only works at about 50% of its theoretic value so your R-40 is really working like R-20 when it is warm.

Quote:

Originally Posted by BlueBSH

one energy auditor wanted to remove the fiberglass and replace it with blown celulose to reduce air flow and heat transfer since it has a higher R value...

Theoretically that is great but obviously the auditor is not paying for the work. Pulling out a bunch of fiberglass to throw it in the dump to put in cellulose is not smart. Over blow the fiberglass with cellulose and you will mitigate most of the convective down grading of the blown in fiberglass and make it work much better.

No sense in throwing it in the dump unless you have money to burn.

Quote:

Originally Posted by BlueBSH

the part of the attic over the 2nd story part is only 4 ft high at the highest point I think it was, so the roof is pretty close to the room's ceiling level, even fully vented, one vent per joist cavity on the roof with the baffels and such, doesn't seme to help much.

You need to air seal, baffle properly, and insulate. Put an R-50 combination of cellulose up there and the home will be worlds more comfortable.

Quote:

Originally Posted by BlueBSH

I am putting in solar fan's to see if that helps, haven't goten to it yet though...

I would say don't bother but if you have them, they can't hurt of the attic floor is properly sealed and air tight.

Quote:

Originally Posted by BlueBSH

the odd thing is we have a 1 story master suite that doesn't get as hot as the 2nd floor... but it has a full attic witch is almost 13ft high at the peak but it doesn't get anywhere near as hot as the 2nd story attic space.. the hottest it was, was 110 degrees at the same time the 2nd story space was 150 degrees... and heat doesn't transfer in that space to the master suite like the 2nd floor one does... same type of blown in insulation also

Lower sloped roofs will not cool as effectively in most cases. Check the venting again and make sure it is as open as you think.

Air seal
Check the vent chutes
Make sure the soffits are as open as you think as are the ridge vents
Insulate up to R-50 with cellulose
Tear up the card of your two auditors

If the temperature is that far off from ambient, it is not vented proper or something else is exacerbating the heat issue. A properly vented attic should only be about 20-25 degrees off of ambient in a sloped roof (4:12 or better)

Your auditor might be a idiot in this case because that is about the most round about way to reduce heat in the attic.

Worst stuff on the market. Usually only works at about 50% of its theoretic value so your R-40 is really working like R-20 when it is warm.

Theoretically that is great but obviously the auditor is not paying for the work. Pulling out a bunch of fiberglass to throw it in the dump to put in cellulose is not smart. Over blow the fiberglass with cellulose and you will mitigate most of the convective down grading of the blown in fiberglass and make it work much better.

No sense in throwing it in the dump unless you have money to burn.

You need to air seal, baffle properly, and insulate. Put an R-50 combination of cellulose up there and the home will be worlds more comfortable.

I would say don't bother but if you have them, they can't hurt of the attic floor is properly sealed and air tight.

Lower sloped roofs will not cool as effectively in most cases. Check the venting again and make sure it is as open as you think.

Air seal
Check the vent chutes
Make sure the soffits are as open as you think as are the ridge vents
Insulate up to R-50 with cellulose
Tear up the card of your two auditors

yeah good point about the soffits, It's just standard vinyal perforated soffit... runs the full length of the eve overhangs, need to pull a few pieces and make sure there isn't fiberglass blocking the baffels or something stupid

when you say air seal, what are you meaning exactly? I had one guy come out and spray foam all the top plates (the big gun 2 part tank type, not the little DIY can stuff) until a couple inches over the drywall to stop air from traveling down the walls, not sure what much else is left to seal.